The present invention relates to a steam turbine having brush seals between non-rotatable and rotatable components arranged and located to eliminate thermal bowing resulting from non-uniform distribution of heat about the rotatable component due to heat generated by frictional contact between the brush seal and the rotatable component and particularly relates to apparatus and methods for eliminating thermal bowing as well as axial thrust loads in the event of failure of the brush seal in such turbine.
In U.S. Pat. No. 6,168,377, of common assignee herewith, there is disclosed a steam turbine having a brush seal located between a non-rotatable component and a rotatable component of the rotor shaft. Particularly, axial flanges are provided on the dovetails of the buckets, the bucket dovetails being secured in complementary fashion to the dovetail of a rotor wheel. A brush seal comprised of an arcuate array of metal bristles projecting from the non-rotatable component toward the rotatable component, i.e., the flanges on the bucket dovetails, has bristle tips engaging with and bearing against the flange surfaces. As will be appreciated from a review of that patent, the contact between the bristles of the brush seal and the opposing sealing surface, i.e., the flanges, generates heat.
As disclosed in that patent, it is recognized that the contact between the brush seal and the sealing surface should be located radially outwardly of the rotor shaft in order to isolate the generated heat from the outer diameter of the rotor. Otherwise, the friction-generated heat may cause a non-uniform temperature distribution about the circumference of the shaft, resulting in non-uniform axial expansion of the rotor and, hence, a bow in the rotor. While various methods and apparatus are disclosed in that patent for eliminating that problem, one such solution locates the friction-generating surface on the bucket dovetail flanges radially outboard of the outer shaft diameter. In that manner, the generated heat is isolated from the rotor, eliminating any tendency of the rotor to bow.
That patented design and other designs utilize conventional labyrinth-type packing seals on the inside of the diaphragm web as a backup to the brush seal. These labyrinth seals are located directly adjacent the outer diameter of the shaft. Brush seals are, however, susceptible to wear and failure. Should a brush seal spaced outwardly from the shaft fail, e.g., the brush seal of that patented design, the sealing diameter changes from the bucket dovetail platform to the rotor shaft. This, in turn, adversely changes the pressure distribution on the shaft and the thrust on the rotor in an axial direction. Thus, rotor dynamic constraints limit the number of stages in which brush seals may be used and where labyrinth-type sealing teeth are used in lieu of such seals, there is a decrease in section efficiency due to increased secondary losses. Accordingly, there is a need to provide a sealing system for a steam turbine in which not only is the problem of thermal bowing of the steam turbine rotor due to non-uniform heat distribution resulting from contact between brush seals and complementary sealing surfaces eliminated, but also the axial thrust loads on the rotor bearings are eliminated or minimized in the event of brush seal failure.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal located radially outwardly of the outer diameter or surface of the shaft of the rotatable component to eliminate thermal bowing of the rotor due to non-uniform heat distribution. The brush seal may be applied in combination with a labyrinth seal at substantially the same radial location to eliminate thrust loads in the event of failure of the brush seal. Particularly, a platform is formed about the rotor between adjacent axially spaced wheels carrying the turbine buckets and which platform projects radially outwardly from the surface of the rotor. The platform, in a preferred embodiment, is in the form of an annular pedestal having an axially reduced neck and at least one and preferably a pair of axially extending flanges at the radial outer extremes of the platform. The one or more flanges are in effect cantilevered in an axial direction from the neck of the pedestal and serve as one or more fins enabling heat generated by frictional contact of the brush seal on the platform surface to be dissipated before affecting rotor dynamics. Thus, the platform configuration enables a sufficient area and provide flanges or fins to dissipate the heat locally, mitigating the effect on rotor vibration, thus allowing similar brush seal application to all steam turbine section stages. It will be appreciated that the cantilevered flange or fin provides a void radially between the flange or fin and the rotor surface, i.e., in the wheelspace, whereby the frictional heat generated by brush seal contact with the sealing surface of the platform is dissipated first in an axial direction and then in a radial direction before having any effect on the thermal dynamics of the rotor. The heat dissipation is sufficient to minimize or eliminate a thermal response of the rotor to the frictionally generated heat.
The diaphragm between the adjacent wheels has a web extending radially inwardly into the wheel space and, not only carries the brush seal, but also one and preferably a plurality of labyrinth seal teeth. The labyrinth seal teeth terminate in tapered edges spaced from a surface of the platform and preferably serve as backup seals to the brush seal. The labyrinth teeth thus are preferably located on the downstream side of the brush seal. Should the brush seal fail, the labyrinth teeth limit performance degradation. The brush seal, however, may be located downstream of the labyrinth seal teeth or intermediate the labyrinth seal teeth. Also, since the areas of the upstream and downstream sides of the pedestal exposed in the cavity integrally of the diaphragm, i.e., the wheelspace, are substantially equal, no net axial thrust from leakage flows past labyrinth seals occurs.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotatable component including a rotor shaft having a rotor shaft surface and a non-rotatable component about the rotatable component, a brush seal carried by the non-rotatable component for sealing engagement with the rotatable component, at least a pair of wheels on the rotatable component spaced axially from one another, the rotatable component including a plurality of buckets spaced circumferentially from one another on each of the wheels, means for inhibiting non-uniform circumferential heat transfer to the rotor shaft surface due to heat generated by frictional contact between the brush seal and the rotatable component thereby to eliminate or minimize bow of the rotatable component, the inhibiting means including an annular platform projecting radially outwardly of the rotor shaft surface at an axial location between the wheels, the brush seal being disposed between the buckets on the wheels and engaging a sealing surface on the platform radially outwardly of the rotor shaft surface.
In a further preferred embodiment according to the present invention, there is provided in a steam turbine having a rotatable component including a rotor shaft having a rotor shaft surface and a non-rotatable component about the rotatable component carrying a brush seal for sealing engagement with the rotatable component along a steam leakage flow path, a method of substantially eliminating bowing of the rotor shaft resulting from circumferential non-uniform distribution of heat about the rotatable component due to heat generated by frictional contact between the brush seal and the rotatable component comprising inhibiting circumferential non-uniform heat transfer to the rotatable component due to heat generated by frictional contact between the rotatable component and the brush seal by locating the area of frictional contact between the rotatable component and the brush seal along a sealing surface spaced radially outwardly of the rotor shaft surface and in radial registration with the rotor shaft surface and a wheelspace portion between the sealing surface and the rotor shaft surface.